Pt mediated C-H activation: Formation of a six membered platinacycle via Csp3-H activation

Article abstract of DOI:10.1016/j.ica.2011.09.030

Herein we report examples of Pt-mediated Csp³-H activation to form 6-membered platinacycles. In one case, Csp³-H activation is preferred over Csp²-F activation. A possible explanation for this unexpected observation is pro

Full text of DOI:10.1016/j.ica.2011.09.030

Inorganica Chimica Acta 380 (2012) 284–290
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Pt mediated C–H activation: Formation of a six membered platinacycle via Csp3-H
activation
⇑
Lauren Keyes, Tongen Wang, Brian O. Patrick, Jennifer A. Love
Department of Chemistry, 2036 Main Mall, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
a r t i c l e i n f o
a b s t r a c t
Herein we report examples of Pt-mediated Csp3-H activation to form 6-membered platinacycles. In one
case, Csp3-H activation is preferred over Csp2-F activation. A possible explanation for this unexpected
observation is provided.
Article history:
Available online 29 September 2011
Young Investigator Award Special Issue
F
F
F
Keywords:
Platinum
C–H activation
Metallacycle
Polyfluoroaryl imines
F
F
F
F
-CH4
Me
N
R
Pt
F
R = H2C
Br
N
R
Me
Me
Pt
PPh3
H3C
PPh3
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
time, a number of important contributions to this field have been
made. Of particular note, Bergman and Graham independently re-
ported some of the first examples of C–H activation of saturated
hydrocarbons utilizing Ir(I) complexes [5a,b,11]. In addition, Pt(II)
complexes have demonstrated particular utility in C–H activation
[7,8]. Importantly, work by Shilov demonstrated that Pt(II) salts
are capable of the oxidation of methane; this remains one of the
only systems for the mild, catalytic functionalization of unactivat-
ed alkanes reported to date [8].1 A variety of other Pt(II) complexes
have also been found capable of activating alkanes and electron rich
arenes [7].
In addition to the high bond-strength of C–H bonds [11], partic-
ularly Csp3-H bonds, selective functionalization is often a consider-
able challenge. Therefore, a common method for achieving selective
C–H functionalization is to utilize a substrate containing a directing
functionality [12]. As a result, much of the recent work on catalytic
C–H activation involves the use of such substrates. The result of
intramolecular C–H activation is the formation of a cyclometalated
complex which can participate in further functionalization, such as
C–C bond formation [12], or be utilized in the synthesis of new
organometallic complexes [12]. For example, cyclometalated
Pt-complexes have been found to have interesting photochemical
properties and potential materials applications [13].
The development of efficient methods for the functionalization
of C–H bonds has been and continues to be a long-standing chal-
lenge in organic and organometallic chemistry. Indeed, the conver-
sion of simple, inexpensive and readily available alkane starting
materials into complex, functionalized products under mild condi-
tions is a powerful synthetic tool with a diverse array of applica-
tions involving the pharmaceutical, agrochemical and petroleum
industries.
To this end, a variety of approaches to C–H activation have been
explored. In particular, transition metals have been extensively uti-
lized in the activation of C–H bonds [1]. Free radicals have also
been employed in the photolytic cleavage of C–H bonds; however
such processes are often plagued by low selectivity and functional
group incompatibility [2]. In comparison, a distinct advantage of
transition metal based C–H activation is that M–C bonds that result
from oxidative addition of C–H
than the starting materials and can be readily converted into
numerous other functionalities [3].
Many different transition metals, including Ru [4], Ir [5], W [6],
Pt [7], Rh [9] and Pd [10], have been found capable of C–H activa-
tion [3]. The first reported C–H activation involved the reaction of
naphthalene with a Ru(0)–diphosphine complex [4a]. Since that
r bond are generally more reactive
During the course of our investigations [14] of the substrate
scope [14a–d] and mechanism [14e] of Pt(II) catalyzed methylation
⇑
Corresponding author.
E-mail address: jenlove@chem.ubc.ca (J.A. Love).
1
For related examples involving Rh complexes see references 8f–l.
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.09.030

L. Keyes et al. / Inorganica Chimica Acta 380 (2012) 284–290
285
Eq. (1), we observed the formation of a 6-membered cyclometalat-
JPt–H = 12.0 Hz, S(CH3)2, 6H), 1.50 (dd, JPt–H = 63.0 Hz, JFe–H
=
9.0 Hz, JFd–H = 6.0 Hz, Pt–CH3, 3H), 0.80 (d, JPt–H = 66.0 Hz,
JFe–H = 6.0 Hz, Pt–CH3, 3H).19F NMR (acetonitrile-d3, 282 MHz): d
ꢀ128.2 (m, 1F), ꢀ139.4 (m, 1F), ꢀ148.0 (m, 1F), ꢀ162.8 (m, 1F),
ꢀ253.5 (tm, JPt–F = 175.1 Hz, 1F). The geometry of this complex
was confirmed by NOE, as described in the Section 3. HRMS (ESI
positive ion mode) calculated for PPh3 complex, prepared in situ
ed Pt(II) complex resulting from the C–H activation of our methyl-
ated imine products. Herein, we discuss the formation and
characterization of Pt(II) complexes resulting from Csp3-H
activation.
F
F
by treatment of
4
with excess PPh3: C44H30NF3P79Br194Pt:
Pt2Me4(SMe2)2 (5 mol%)
NR'
NR'
820.0414. Found: 820.0414.
Me2Zn (0.6 equiv), CH3CN
Me
R
70- 95% yield
ð1Þ
R
F
60°C, 8 h
2.2.2. [PtMe3(PPh3)(C6F4CH@NCH2C6H4Br)], complex 6
R = F, Br, Cl, CN
R' = -CH2Ar or Ar
In a 20 mL vial inside the glovebox, N-(2,3,4,5,6-pentafluorob-
enzylidene)para-bromobenzylamine
(0.063 g,
0.174 mmol,
1.0 equiv) and Pt2Me4(SMe2)2 (0.050 g, 0.087 mmol, 0.5 equiv)
were dissolved in 1.1 mL CD3CN. The resulting solution was left
at room temperature for 24 h after which 1.2 equiv. Me2Zn
2. Experimental
(102 lL, 0.204 mmol, 2.0 M solution in toluene) was added via syr-
2.1. General procedures
inge. The resulting solution was left at room temperature for an-
other 20 min and was, subsequently, combined with PPh3
(0.045 g, 0.174 mmol, 1.0 equiv). An orange precipitate formed
after 24 h at room temperature. The solid was separated by filtra-
tion through a glasswool plug, washed with ꢁ2 mL of pentanes and
transferred to a separate 20 mL vial. The solid was dissolved in
ꢁ2 mL of dichloromethane and carefully layered with 8 mL of pen-
tanes. When the solution was left at room temperature for 24 h,
red crystals formed in 40% yield. The geometry of this complex
was confirmed by X-ray structural analysis and is consistent with
solution state NOE experiments. See Supporting information for
details.1H NMR (acetone-d6, 300 MHz): d 8.59 (s, JPt–H = 40.5 Hz,
CH@N, 1H), 7.54 to 7.04 (m, overlapping peaks, aryl-H), 4.55 (d),
4.23 (d, JH–H = 15.3 Hz, CH2, 2H, AB pattern), 1.37 (dd,
JPt–H = 70.8 Hz, JF–H = 2.7 Hz, JP–H = 7.7 Hz, Pt–CH3, 3H), 0.62 (d,
JPt–H = 49.8 Hz, JP–H = 7.3 Hz, Pt–CH3, 3H), 0.42 (d, JPt–H = 62.0 Hz,
JP–H = 7.5 Hz, Pt–CH3, 3H). 19F NMR (acetone-d6, 282 MHz): d
ꢀ124.4 (m, 1F), ꢀ140.2 (t, J = 19.7 Hz, 1F), ꢀ149.8 (m, 1F),
ꢀ165.2 (t, J = 19.7 Hz, 1F). 31P{1H} NMR (acetone-d6, 121 MHz): d
ꢀ6.94 (s, JPt–P = 1039.2 Hz). 13C{1H} NMR (CD2Cl2, 100 MHz): d
Manipulation of organometallic compounds was performed
using standard Schlenk techniques under an atmosphere of dry
nitrogen or in a nitrogen-filled Vacuum Atmospheres drybox
(O2 < ppm). NMR spectra were recorded on Bruker Avance 300,
Bruker Avance 400 or Bruker Avance 600 spectrometers. 1H
and 13C chemical shifts are reported in parts per million and ref-
erenced to residual solvent. Coupling constant values were ex-
tracted assuming first-order coupling. The multiplicities are
abbreviated as follows: s = singlet, d = doublet, t = triplet,
m = multiplet. All spectra were obtained at 25 °C. Mass spectra
were recorded on a Kratos MS-50 mass spectrometer. Elemental
analyses were performed using a Carlo Erba Elemental Analyzer
EA 1108. Single X-ray structure determinations were performed
by Dr. Brian O. Patrick at the Department of Chemistry, Univer-
sity of British Columbia utilizing ADSC, Rigaku AFC7, or Bruker
X8 Apex CCD area detectors with graphite-monochromated Mo
Ka radiation.
2.2. Materials and methods
169.2 (br. s), 154.7 (dm, JBr–C = 43.3 Hz), 151.8 (dm, JF–C
=
230.5 Hz), 148.8 (dm, JF–C = 261.6 Hz), 143.5 (dm, JF–C = 265.7 Hz),
Acetonitrile and dichloromethane were dried by heating to re-
flux over calcium hydride, stored over molecular sieves and de-
gassed prior to use. Pentanes and hexanes were purified and
dried by passage through a column of activated alumina and
sparged with dinitrogen prior to use. Acetonitrile-d3 was obtained
from Cambridge Isotopes, Inc. and degassed prior to use. All organ-
ic reagents were obtained from commercial sources and used as
received. 1,3,5-Trimethoxybenzene was sublimed prior to use.
K2PtCl4 was purchased from Strem Chemicals and was used with-
out further purification. PtCl2(SMe2)2 and Pt2Me4(SMe2)2 were
prepared by a previously reported procedure [15]. All imines were
prepared based on previously reported procedures [14]. The
characterization data for all imines is given in the Supporting infor-
mation. Dimethyl zinc (1.2 M solution in toluene) was purchased
from Aldrich and titrated with LiCl and I2 prior to use [16].
137.2 (m, overlapping with resonances for PPh3), 134.3 (d, JPt–C
9.9 Hz), 136.2 (s), 132.1 (s), 131.8 (d, JPt–C = 10.8 Hz, JPt–C
=
=
37.5 Hz), 130.7 (d, JPt–C = 2.3 Hz), 128.6 (d, JPt–C = 8.4 Hz), 122.9
(s), 59.2 (s, JPt–C = 14.5 Hz), 7.29 (d, JPt–C = 543.4 Hz, JPt–C
=
114.7 Hz), 2.17 (br. s, JPt–C = 478.0 Hz), ꢀ12.9 (dd, JPt–C = 613.9 Hz,
JPt–C = 11.1 Hz, JF–C = 4.0 Hz). HRMS (EI) m/z calc. for
C
35H30NF4P79Br194Pt (M–H): 844.0862; found: 844.0850. Anal. Calc.
for C35H31NF4PBrPt: C, 49.60; H, 3.69; N, 1.65. Found: C, 49.80; H,
3.72; N, 1.75.
2.2.3. [PtMe{CH2C6F4CH@NCH2C6H5}PPh3], complex 7
When crystals of Me3Pt(IV)–PPh3 complex (6) were left in the
recrystallization solvent for more than 2 days, they re-dissolved
and resulted in precipitate formation. The mixture was filtered
through a glasswool plug to provide a clear solution which was
concentrated to dryness. The residue was dissolved in CD3CN and
brown crystals formed after 2 weeks at room temperature in 15%
yield. The structure of the complex was confirmed by X-ray struc-
tural analysis.
2.2.1. [PtMe2F(SMe2)(C6F4CH@NCH2C6H4Br)], complex 4
In
a 20 mL vial, N-(2,3,4,5,6-pentafluorobenzylidene)para-
bromobenzylamine (0.034 mmol, 12.6 mg, 1.0 equiv) and
Pt2Me4(SMe2)2 (0.017 mmol, 9.8 mg, 0.5 equiv) were dissolved in
CD3CN (1.0 mL). The resulting solution was left at room tempera-
ture for 16 h and then transferred to an NMR tube fitted with a
screw cap containing a septum. Analytical data is based on in situ
NMR spectroscopic characterization. >90% Conversion based on
integration of the 19F NMR spectrum. 1H NMR (acetonitrile-d3,
300 MHz): d 8.98 (s, JPt–H = 48.0 Hz, CH@N, 1H), 7.67–7.45 (m,
resonances of arylprotons, 4H), 5.02 (m, CH2, 2H), 1.96 (s,
2.2.4. [PtMe2F(SMe2)(C6F3CH3CH@NCH2C6H5)], complex 8
In
a
20 mL vial, N-(3,4,5,6-tetrafluoro-2-methylbenzylid-
ene)benzylamine (2) (0.034 mmol, 11.5 mg, 1.0 equiv) and
Pt2Me4(SMe2)2 (0.017 mmol, 9.8 mg, 0.5 equiv) were dissolved in
CD3CN (1.0 mL). The resulting solution was transferred into an
NMR tube, which was then fitted with a screw cap containing a
septum. The reaction was monitored by 1H and 19F NMR spectros-

286
L. Keyes et al. / Inorganica Chimica Acta 380 (2012) 284–290
copy at room temperature over 24 h. The geometry of this complex
was determined on the basis of the magnitude of Pt–H coupling.
See Supporting information for details. Analytical data is based
on in situ NMR spectroscopic characterization. 83% conversion
based on integration of the 19F NMR spectrum. 1H NMR (acetoni-
trile-d3, 300 MHz): d 9.10 (s, JPt–H = 11.7 Hz, CH@N, 1H), 7.76 to
complex was determined on the basis of the magnitude of Pt–H
coupling. See Supporting information for details. Analytical data is
based on in situ NMR spectroscopic characterization. >90% Conver-
sion relative to the amount of complex 9. 1H NMR (toluene-d8,
300 MHz): d 8.24 (d, JPt–H = 18.0 Hz, CH@N, J = 2.1 Hz), 7.75–7.44
(m, overlapping signals, aryl resonances), 4.90 (d, JH–H = 1.8 Hz,
CH2, 2H, AB pattern), 2.66 to 2.63 (m, CH2–Pt, 2H), 2.25 (s, CH3,
3H), 1.66 (d, JPt–H = 69.6 Hz, Pt–CH3, 3H). 19F NMR (toluene-d8,
282 MHz): d ꢀ138.1 (d, J = 25.1 Hz, 1F), ꢀ140.3 (dd, J = 20.6 Hz, J =
11.6 Hz, 1F), ꢀ174.0 (m, 1F). 31P{1H} NMR (toluene-d8, 262 MHz): d
28.2 (s, JPt–P = 4041.4 Hz). HRMS (ESI) positive ion mode m/z Anal.
Calc. for C34H29NF3P194Pt (M–CH3): 733.1617. Found: 733.1612.
7.04 (m, overlapping signals, aryl resonances), 4.06 (d, JH–H
7.5 Hz, CH2, 2H, AB pattern), 2.55 (s, CH3, 3H), 2.00 (s, JPt–H
=
=
3.6 Hz, S(CH3)2, 6H), 1.67 (d, JPt–H = 64.8 Hz, Pt–CH3, 3H), 1.08 (d,
JPt–H = 67.8 Hz, Pt–CH3, 3H).19F NMR (acetonitrile-d3, 282 MHz): d
ꢀ147.3 (dd, J = 19.7 Hz, J = 11.3 Hz, 1F), ꢀ147.9–148.0 (m, overlap-
ping signals, aryl resonances), ꢀ153.5 (d, JPt–H = 41.2 Hz, J = 4.5 Hz,
1F), ꢀ245.4 (t, J = 7.1 Hz, 1F). HRMS (ESI positive ion mode) calcu-
lated for PPh3 complex, prepared in situ by treatment of 8 with ex-
cess PPh3: Anal. Calc. for C34H29NF3P194Pt (M-2CH3): 733.1630.
Found: 733.1617.
3. Results and discussion
We recently reported the first examples of Pt(II) catalyzed C–F
cross-coupling of polyfluoroarenes [14]. These methodologies uti-
lize a bis-platinum dimer – Pt2Me4(SMe2)2 – to facilitate activation
of a C–F bond with further functionalization at carbon. For exam-
ple, imine (1) undergoes monomethylation with high selectivity
to generate (2) (Scheme 1). Addition of excess Me2Zn can result
in a second ortho-methylation if the fluoroarylimine is sufficiently
electron deficient; such is the case with imine (2), which can form
the corresponding dimethylated product (3).
Mechanistic investigations have shown the reaction to proceed
through a standard cross-coupling mechanism involving C–F acti-
vation has an initial step after which transmetalation and reductive
elimination result in methylation ortho to the imine (Scheme 2)
[14d].
Throughout the course of our investigations into the mecha-
nism of catalytic methylation we sought to examine the stoichiom-
etric reactivity of catalytically relevant Pt(IV) complexes as a
means of better understanding the fundamental steps involved in
catalysis. Of particular interest was the preparation and isolation
of a Me3Pt(IV) complex given its importance in the catalytic cycle.
To this end, we determined that Pt(IV)–F complex (4), prepared
from C–F activation of pentafluoroimine (1), could undergo trans-
metalation with Me2Zn to generate Me3Pt(IV) complex (5). Due
to the low stability of Pt(IV)fluoroaryl SMe2 complexes [17a], li-
gand exchange with PPh3, to give Me3Pt(IV)ꢂPPh3 complex (6),
was required for solid state characterization Eq. (2). The low stabil-
ity of such Pt(IV)ꢂSMe2 complexes is likely the result of the lability
of the SMe2 ligand [17a]. As a result, Pt(IV)ꢂSMe2 complexes were
characterized in situ using 1H and 19F NMR spectroscopy.
2.2.5. [PtMe{CH2C6F3CH3CH@NCH2C6H5}SMe2], complex 9
In a 20 mL vial, N-(2,6-difluoro-3,4,5-trifluorobenzylidene)ben-
zylamine (3) (0.034 mmol, 9.7 mg, 1.0 equiv) and Pt2Me4(SMe2)2
(0.017 mmol, 9.8 mg, 0.5 equiv) were dissolved in toluene-d8
(1.0 mL). The resulting solution was transferred to an NMR tube,
which was then fitted with screw cap containing a septum. The
reaction was monitored, at the indicated temperature, by 1H and
19F NMR spectroscopy over 24 h. The geometry of this complex
was determined on the basis of the magnitude of Pt–H coupling
as well as NOE experiments. See Supporting information for de-
tails. Analytical data is based on in situ NMR spectroscopic charac-
terization. 19% conversion based on integration of the 1H NMR
spectrum.1H NMR (toluene-d8, 300 MHz):
d 7.73 (s, JPt–H =
11.4 Hz, CH@N, 1H), 7.30–6.89 (m, overlapping peaks, aryl reso-
nances), 4.36 to 4.34 (d, JH–H = 2.1 Hz, CH2, 2H, AB pattern), 2.10
to 2.06 (m, overlapping with solvent, 5H), 1.87 (s, JPt–H = 11.4 Hz,
S(CH3)2, 6H), 0.90 (s, JPt–H = 43.5 Hz, Pt–CH3, 3H). 19F NMR (tolu-
ene-d8, 282 MHz): d ꢀ139.5 (d, J = 20.6 Hz, 1F), ꢀ152.5 (dd,
J = 25.4 Hz, J = 6.8 Hz, 1F), ꢀ178.5 (m, 1F).
2.2.6. [PtMe{CH2C6F3CH3CH@NCH2C6H5}PPh3], complex 10
In a 20 mL vial, PPh3 (0.034 mmol, 8.9 mg, 1.0 equiv) was
weighed. A solution of complex 9 was added and swirled until com-
plete dissolution. The resulting solution was transferred into an
NMR tube, which was then fitted with a screw cap containing a sep-
tum. The reaction was monitored by 1H, 19F and 31P{1H} NMR spec-
troscopy at room temperature over 24 h. The geometry of this
Scheme 1. Functionalization of pentafluoroaryl imine.

L. Keyes et al. / Inorganica Chimica Acta 380 (2012) 284–290
287
Table 1
Crystallographic data for complexes 6 and 7.
6
7
Formula
C
35H31NF4PPtBr
C34H27NF4PPtBr
831.54
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
847.58
triclinic
P1
monoclinic
P21/c (#14)
8.4349(4)
21.6442(10)
16.9321(7)
90.0
101.601(1)
90.0
3028.1(2)
4
ꢀ
10.5281(7)
10.5282(7)
15.3290(15)
86.304(3)
76.090(3)
69.947(3)
1548.98(18)
2
a
(°)
b (°)
c
(°)
V (A3)
Z
Dcalc (g cmꢀ3
T (K)
)
1.817
173
1.824
173
1
Solvent/colour
acetonitrile/
colourless
824.00
acetonitrile/
brown
1608.00
55.8
F(000)
2hmax
56.2
Total number of reflections collected 33132
32500
Number of unique reflections
(F > 4 (F))
Rint
7494
7213
r
0.036
0.033
Scheme 2. Mechanism of Pt(II) catalyzed methylation of polyfluoroarenes.
R1, wR2 [I > 2s(I)]
0.017, 0.040
1.04
0.74, ꢀ0.88
0.026, 0.051
1.09
0.88, ꢀ1.38
Goodness of fit (GOF)
Peak/hole (e Aꢀ3
)
Fig. 1. ORTEP representation of the solid state molecular structure of 6 (ellipsoids
Fig. 2. ORTEP representation of the solid state molecular structure of 7 (ellipsoids
plotted a0t 50% probability, hydrogens removed for clarity) with selected bond
lengths (AÅ) and torsion angles (deg): Pt – N1, 2.135(3); Pt – P1, 2.2719(8); Pt – C1,
2.052(3); Pt – C36, 2.077(3). N1 – Pt – P1, 100.60(7); C1 – Pt – P1, 89.66(10); C1 – Pt
– C36, 86.92(13).
plotted at 50% probability, hydrogens removed for clarity) with selected bond
0
lengths (AÅ) and torsion angles (deg): Pt – C1, 2.076(2); Pt – C2, 2.095(2); Pt – C3,
2.057(2); Pt – N1, 2.1435(17); Pt – P1, 2.3743(5); Pt – C22, 2.099(2); C3 – Pt – C22,
97.49(9); C22 – Pt – N1, 78.81(8); C2 – Pt – N1, 93.33(8); C3 – Pt – C2, 89.43(10).
F
F
F
F
F
F
F
F
F
PPh3 (1.0 equiv)
Me2Zn (0.6 equiv)
F
F
F
Me
Me
Me
N R
Me
N R
F
N R
Me
r.t, 24 h, CH3CN
r.t, 20 min, CH3CN
> 90 % conversion
Pt
Pt
Pt
Me
Me
Me
isolated yield: 40 %
ð2Þ
PPh3
SMe2
SMe2
4
5
6
R =
R =
Br
Br
H2C
R =
H2C
Br
H2C

288
L. Keyes et al. / Inorganica Chimica Acta 380 (2012) 284–290
Scheme 3. Endo vs. exo metallacycles.
On the other hand, crystals suitable for solid state characteriza-
Pt2Me4(SMe2)2 results in C–F activation as the only organometallic
product Eq. (3). Although we cannot comment on whether any C–F
activation product was formed from the crystals of 6, it is surpris-
ing that C–H activation occurred at all.
Several mechanisms that account for conversion of 6–7 are pos-
sible. Pt(II) complex (7) could be formed as a result of a two step
process; reductive elimination from Me3Pt(IV)ꢂPPh3 complex (6)
would generate methylated imine product (2), which then would
undergo Csp3-H activation. A similar mechanism for the formation
of related Pt(II) 6-membered endo-metallacycles was proposed by
Crespo [17h]. It is possible that Pt remains coordinated to the imine
after reductive elimination. The presence of an agnostic interaction
between the methyl C–H bond and Pt would account for the selec-
tion of Pt(IV)ꢂPPh3 complex (6) were obtained by layering a satu-
rated dichloromethane solution with pentanes at room
temperature. The solid state molecular structure of Me3Pt(IV)ꢂPPh3
complex (6) is depicted in Fig. 1. Crystallographic parameters for
complex (6) are included in Table 1. This complex was also charac-
terized by multinuclear NMR spectroscopy, HRMS and elemental
analysis. Full characterization details are included in the experi-
mental section.
During the course of our preparation and isolation of Me3P-
t(IV)ꢂPPh3 complex (6) we observed the formation of an unex-
pected Pt(II) complex. The crystals of complex (6) re-dissolved
upon standing; after several weeks at room temperature new crys-
tals suitable for solid state characterization were formed. X-ray
analysis of the crystalline material revealed a cyclometalated Pt(II)
complex (7). The solid state molecular structure of this complex is
depicted in Fig. 2. Unfortunately, additional attempts to crystallize
7 failed and thus only X-ray data are available. More complete
characterization data are given for a related complex (10) whose
synthesis is reproducible (vide infra).
As shown in Fig. 2, Pt(II) complex (7) is a 6-membered metalla-
cycle which is formed as a result of Csp3-H activation. The selectiv-
ity for formation of (7) merits further discussion. Cyclometalation
of Csp2-H bonds is typically preferred over Csp3-H bonds and for-
mation of 5-membered metallacycles is typically preferred over
6-membered metallacycles (Scheme 3) [18].
Indeed, to date only a few examples of metal mediated C–H
activation resulting in the formation of 6-membered metallacycles
have been reported. The majority of such examples involve Pt
[17h,19], Au [20], Pd [21], Co [22] and Ir [23] complexes. The
majority of platinacycles consist of 5- or 7-membered cyclometa-
lated ligands [19]. Both we and Crespo [17h] observe preferential
activation of the methyl Csp3-H bond to form a 6-membered
metallacycle rather than activation of a Csp2-H bond. Crespo sug-
gests that the selectivity arises from the position of the imine
unsaturation relative to the metalation step; endo-metalation is
typically preferred over exo-metalation [17b]. This would explain
the observed preference for C–H activation.
tive C–H activation. Addition of a C–H
r bond across a metal centre
proceeding via initial formation of an agostic interaction has been
extensively documented [24]. An alternate mechanism involves
activation of the Csp3-H bond followed by a [1,5]-sigmatropic shift
to generate a quinodimethane intermediate. Subsequent reductive
elimination of methane from a complex of this type would also
lead to the formation of complex 7. At present, we do not have suf-
ficient evidence to favour these possible mechanisms over other
possibilities.
F
F
CH3
F
F
Pt2Me4(SMe2)2 (0.5 equiv)
NBn
CH3
F
CD3CN, r.t, 24 h
83 % conversion
Me
Pt
NBn
F
F
F
Me
SMe2
2
8
ð3Þ
Attempts to promote the C–H activation of mono-methylated
imine (2) proved unsuccessful and resulted in exclusive C–F activa-
tion Eq. (3). Therefore, we sought to explore the potential for
N-(2,6-dimethyl-3,4,5-trifluorobenzylidene)benzylamine (3) to
undergo C–H activation with the Pt-dimer. The presence of two
orthomethyl groups serves to block any potential C–F activation
and drive the reaction towards Csp3-H activation. Indeed, Csp3-H
activation was found to proceed at room temperature resulting
in formation of Pt(II) complex (9) Eq. (4). While the reaction pro-
ceeded slowly at room temperature, requiring 24 h to proceed to
It is particularly remarkable that we observe selective activation
of a C–H functionality over an aryl C–F bond, particularly as the lat-
ter would form a 5-membered ring and would also be an endo-
metalation. Moreover, treatment of methylated imine (2) with

L. Keyes et al. / Inorganica Chimica Acta 380 (2012) 284–290
289
Table 2
1H NMR spectroscopic data for complex (9).
Entry
d
Multiplicity
Relative intensity
Assignment
1
2
3
4
5
7.73
4.35
2.10–2.06
1.87
0.90
s (JPt–H = 11.4 Hz)
1
2
2
6
3
CH@N
CH2
CH2–Pt
S(CH3)2
Pt–CH3
d (JH–H = 2.1 Hz, AB pattern)
m (overlapping with solvent)
s (JPt–H = 11.4 Hz)
s (JPt–H = 43.5 Hz)
17% conversion, any increase in temperature resulted in rapid
decomposition of the Pt complex. The reaction was monitored by
1H and 19F NMR spectroscopy. Diagnostic chemical shifts of the
crude reaction product are listed in Table 2.
In summary, we report the isolation and characterization of an
unusual 6-membered platinacycle resulting from reductive elimi-
nation and C–H activation of a Me3Pt(IV) complex. The activation
of a Csp3-H bond to form a 6-membered metallacycle instead of
Csp2-F activation to form a 5-membered metallacycle is particu-
larly remarkable. Additional investigations revealed the potential
for di-methylated polyfluoroaryl imines to undergo C–H activation
generating similar 6-membered cyclometalated Pt(II) complexes.
Detailed investigations of the reactivity of related Pt(IV) com-
plexes, particularly transmetalation and reductive elimination
steps are underway.
F
CH3
F
F
CH3
F
F
Pt2Me4(SMe2)2 (0.5 equiv)
NBn
CH3
toluene-d8, r.t, 24 h
ð4Þ
NBn
CH3
17 % conversion
F
Pt
Me2S
3
9
Acknowledgments
The formation Pt(II) complex (9) was clearly indicated by the
presence of a multiplet at d 2.10 to 2.06 which was assigned as a
CH2–Pt bond based on similar values obtained by Crespo (entry
3; Table 2)[17h]. In addition, the formation of another multiplet
at d 4.35 with an AB splitting pattern is consistent with formation
of a cyclometalated imine ligand (entry; Table 2). Upfield singlets
at d 1.87 and d 0.90 (entries 4–5; Table 2) with Pt–H splitting con-
firm the presence of Pt–S(CH3)2 and Pt–Me ligands (entries 4–5,
respectively).
The geometry of the complex was assigned on the basis of the
magnitude of Pt–H coupling as well as NOE data2. Specifically,
the magnitude of the Pt–H coupling of the Pt–CH3 ligand is consis-
tent with a methyl group trans to an X-type ligand (herein, the
X-type ligands are the benzyl and methyl functionalities). This
geometry was confirmed by NOE data which shows contacts be-
tween the Pt–CH3 ligand and signals associated with the methy-
lene linker and benzyl ring. Furthermore, preparation of the
analogous Pt(II)ꢂPPh3 complex (10) was necessary for additional
characterization, including HRMS. The displacement of the SMe2
ligand was monitored by 1H, 19F and 31P{1H} NMR spectroscopy
Eq. (5).
We thank the following for support of this research: NSERC
(Collaborative Research and Development Grant, Research Tools
and Instrumentation Grants), AstraZeneca Canada (2008 Award
in Chemistry) and the University of British Columbia.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.09.030.
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